1. Field of the Invention
The present invention relates to electronic devices having inductors and capacitors disposed in multi-layer wiring boards. In particular, the present invention relates to a low-pass filter with a trap at high frequencies.
2. Description of the Related Art
Low-pass filters can be employed to eliminate unnecessary frequency components generated at frequencies higher than a target frequency band. For example, when a transmitting circuit of a mobile phone transmits a signal from an antenna after performing power amplification on the signal by a power amplifier, unnecessary frequency components are generated at frequencies twice that of and three times greater than a desired frequency used for the signal transmission. As shown in FIG. 4, a low-pass filter having a stop band set at frequencies higher than the desired frequency can efficiently eliminate the unnecessary frequency components by having a trap at frequencies corresponding to the unnecessary frequency components.
FIG. 5 shows an example of a configuration of an LC low-pass filter with a trap. The LC low-pass filter shown in FIG. 5 is a T-section filter. More specifically, the LC low-pass filter has inductors L1 and L2 serially connected to a signal line between an input terminal VIN and an output terminal VOUT and a capacitor C connecting the connecting point of the inductors L1 and L2 and a ground. An inductor L3 is connected between the connecting point of the inductors L1 and L2 and the capacitor C so as to constitute an LC series resonant circuit between the signal line and the ground. In such an LC low-pass filter, setting a resonant frequency of the LC series resonant circuit at a trap frequency allows the frequency component at the trap frequency to be attenuated as shown in FIG. 4.
A multi-layer wiring board has tended to include the above-described low-pass filter and other circuit elements therein due to miniaturization and integration of electronic devices having the above-described low-pass filter.
FIG. 6 is a conceptual diagram showing a cross-sectional view of a multi-layer wiring board constituting the above-described low-pass filter. The multi-layer wiring board includes a plurality of laminated dielectric layers (not shown). Each dielectric layer has conductive patterns or the like formed thereon. First to third conductive patterns 101 to 103 are formed on adjacent dielectric layers of the multi-layer wiring board. In addition, fourth to sixth conductive patterns 104 to 106 are formed next to the first to third conductive patterns 101 to 103, respectively, on the corresponding dielectric layers. The first to third conductive patterns 101 to 103 formed on the plurality of dielectric layers constitute an inductor L1. Likewise, the fourth to sixth conductive patterns 104 to 106 constitute an inductor L2. Seventh and eighth conductive patterns 107 and 108, which constitute an inductor L3, are formed on dielectric layers below those having the inductor L2 (i.e., the fourth to sixth conductive patterns 104 to 106). A via hole 110 conductively connects one end of the sixth conductive pattern 106 and a part of the seventh conductive pattern 107. In addition, an electrode layer 120, i.e., a ground, formed on the lowest dielectric layer of the multi-layer wiring board and an electrode 109 formed on a dielectric layer immediately above the lowest dielectric layer constitute a capacitor C. A via hole 111 conductively connects the electrode 109, i.e., one of the electrodes of the capacitor C, and the eighth conductive pattern 108.
As described above, the inductors L1, L2, and L3 and the capacitor C included in the multi-layer wiring board constitute a low-pass filter having frequency characteristics in which the frequency component at the trap frequency is attenuated as shown in FIG. 4.